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How to perform a "first principle" performance calculation?
I am a PhD student on flight dynamics and control. This week I was given an assignment to do some research on methods of aircraft performance calculation. After some search on the Internet, I found that traditional methods depended on tabulated data on the airplane flight manual (APM), and about a decade ago, both Boeing and Airbus started to do performance calculation using "first principle" method.
I read on an Advisory Circular document that provides a definiton of this method:

A calculation using basic parameters such as lift, drag, power or thrust, etc. with the equations of motion. (AC No: 25.1581-1, Change 1.)

And an Airbus document gives some explanation on this method:

The next step in the performance calculation process, referred to as OCTOPUS (Operational and Certified Takeoff and landing Universal Software), not only offers the same advantages as TLC but also drastically changes the performance calculation method. It is no longer based on pre-computed data, but uses the “first principle” mode that allows a real on-time computation to benefit from a higher takeoff weight. Instead of smoothed pre-computed performance results, the OCTOPUS performance database contains all the airplane and engine characteristics, enabling performance computation based on physics equations. In addition, OCTOPUS introduces a new and improved takeoff chart format, with its use of multi-configurations and influences. ("Getting to Grips with Aircraft Performance", Airbus)

However, I have not found any paper or report on this topic. Is there anybody out there who could help by offering some details of this method, or, providing some references?

The equations of motion are the easy part. In essence, you look at all forces affecting the aircraft (lift, thrust, drag, weight) and balance them with proper control settings (elevator, throttle) and accelerations (if thrust > drag, the forward acceleration is (thrust - drag)/mass).
This you repeat over and over, one timestep at a time. The next timestep sees the aircraft at a new speed, which you get by multiplying the forward acceleration with time, and altitude if the climb speed is nonzero. The new, changed mass is the old mass minus the fuel consumed during the last timestep. And so on. This involves coordinate translations as some forces are defined in the aerodynamic and others in the airplane coordinate system. NASA Langley has published an open source software which does exactly that (LaRCsim).
For very high precision you can even model the inertias and calculate which aileron deflection is needed to arrive at a desired roll rate in the next timestep, but even without that you will get very precise data if the forces are correct.
The hard part is to arrive at the correct forces. We have had several questions here asking for the aerodynamic data of modern airliners, and every time the answer was: They are kept secret. You need to do your own analysis, and it is the same with engine data. Older methods relied on tabulated data, but in order to calculate conditions outside of the validity of the table they need to calculate the forces analytically. To get an idea what parameters need to be considered, look at this answer about the Boeing SCAP module.
However, even some crude assumptions can get you very close to the real result.

I find that basic lift/drag data is surprisingly accurate for (flight) performance in Obert’s “Aerodynamic Design of Transport Aircraft”, however I haven’t found similar open data on engine performance, mainly thrust/fuel flow relation. I am not sure how close one can get by simulation of the engine gas dynamics equations using similar open data. For (certified) takeoff and landing performance, additional considerations come into play, too, as that depends also on spoiler, brake and even test pilot characteristics.

Thank a lot, Peter.
I am familiar with the modelling of aircraft dynamics and flight simulation since they are required for the validation of the control system design. As for the modelling and the subsequent performance calculations/simulations, aerodynamic data and the engine model are indeed crucial, though not easy to obtain (if not unattainable). For now, I am trying to figure out how to perform a "first principle" performance  calculation, i.e. the procedures and methods, etc. And actual calculation of a specific type of aircraft is out of my current scope.

Yesterday I searched the web and found that Jeppesen and Boeing both proposed a performance calculation tool (Jeppesen/Boeing), in Jeppesen's brochure, it is explicitly stated that 'first principle' method is employed. I guess it is the same case with the Boeing product, since 'real-time' calculation is included as a feature.

I also searched Engineer Village and Web of Knowledge (two big journal database), however, I could not find any academic paper or report explaining the specific techniques required for such calculations, which seems weird. I wonder if it is already covered in classical flight mechanics textbooks. Maybe I need to hit the books again...

@Tomas: Yes, textbooks should cover the topic well. The Internet cam too late, so what you need is still mostly in books. For a database on drag and lift I recommend the books by Sighart Hoerner. However, you should indeed pick a specific type or create a generic one. In the end you need the actual forces, masses, thrust and fuel consumption data. For engine simulation use Gasturb - I always produced an engine deck from it and scaled that up and down when parameters changed in my models.

@PeterKämpf Hmm, I see. Thank you very much for your help.

It's the aerodynamic and inertial data also used in Full Flight Simulators (FFS). Both Airbus and Boeing provide the data package for Level D FFS, where at the beginning of the flight the instructor enters payload, CoG, fuel load etc and the resulting aircraft dynamics are used for simulating flight.
You could have a look in open source PC flight simulator software such as FlightGear, which has aerodynamics and flight dynamics models for both A320 and B737. Simplified and not certified of course so no reference to actual fidelity, but they should get a reasonable result if the simulated dynamics are in the ballpark.

Thanks for your answer. So I guess you mean the main difference lies in the data utilized to perform the calculation, whereas the basic calculation method is the same, either in the traditional way based on tabulated data, or the 'modern' way using "first principle" method. Is that the case?

"A calculation using basic parameters such as lift, drag, power or thrust, etc. with the equations of motion". The "equations of motions" bit is clear, the "basic parameters" can cause a bit of confusion in this. The old method reference consisted of tabulated data. The First Principle uses the result of equations of motion as reference. Both the old method and the First Principle method can use the same aerodynamic data as input, probably also in the form of tabulated data, with the First Principle delivering much higher granularity and accuracy. Many more Degrees of Freedom available.

I understand your point, using equations of motion as an analysis tool provides more fruitful results and capabilities in performance calculation. Have you ever read any paper or report explaining specifics of using the First Principle Method?

I am familiar with the same methods from working on the source code for aerodynamics, engines and flight dynamics models of Full Flight Simulators. I haven't read a paper explaining specifics of First Principle, however from the definition I understand that it is exactly identical to FFS.

Your work sounds cool!
Thank you for your reference, I'll go to find some papers in the FFS area to read.

The only way to properly do a „first principle“ calculation is by (numerically) evaluating the basic equations of motion found in flight mechanics text books. Unfortunately this requires a lot of information on aircraft characteristics (e.g. drag data or engine parameters) that generally aren’t published for most aircraft due to their business sensitivity. Sidenote: Estimates can get you pretty far!
Manufacturer provided software like OCTOPUS (Airbus) or BPS (Boeing) contains a method of numerically evaluating the (often simplified) equations of motion and the required aircraft databases but aren’t publicly available to the best of my knowledge.

Yeah, I understand that to achieve a certain degree of accuracy, information as aerodynamic coefficients and engine models are necessary. But for now, I am interested in the specific techniques/procedures required to carry out "First Principle" calculations. Yesterday I searched Engineer Village and Web of Knowledge (two big journal database), however, I could not find any academic paper or report explaining the specific techniques, which seems weird. I wonder if it is already covered in classical flight mechanics textbooks.


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Why does the LP compressor rotate in the opposite direction of the HP compressor?
Why does the LP compressor rotate in the opposite direction of the HP compressor in some multi-spool turbine engines?

most likely to balance the gyro effects. Do you have an example of such an engine?

I'd like an example as well. All engines I know of, 2-spool or 3-spool, rotate in the same direction to minimize wear between stages.

Related: https://aviation.stackexchange.com/questions/31613/why-are-contra-rotating-jet-engines-so-rare

Honeywell TFE731 engines have spools that rotate in opposite directoins.

Contra-rotating spools are a very recent development, apart from the Bristol-Siddeley (later: Rolls-Royce) Pegasus engine for the Harrier. The Rolls-Royce Trent 900 started the trend when its high pressure spool was made to rotate in the opposite direction to the other two spools. From Wikipedia:

It is also the first member of the Trent family to feature a
  contra-rotating HP spool

Next was General Electric with the GEnx, a two-spool design with a high pressure compressor rotating clockwise while the low pressure compressor rotates counterclockwise.
Contra-rotation means that the relative speeds between the spools are increased, increasing the bearing losses between them. However, contra-rotation helps to increase efficiency - now the stator between the last low pressure stage and the first high pressure stage of the compressor and the stator between the last high pressure turbine stage and the first low pressure turbine stage can be left away which reduces parts count, engine length, engine mass and friction.
Again Wikipedia:

Counter-rotating spools for the reaction turbines to reduce load on
  guide vanes.

I've heard somewhere that contra-rotation is done to counter the gyroscopic precision by the force ( thrust ) compressors have made , and as you told in your description the interacting stator vane rows have been omitted but what is the reason of this elimination?

@Mehdishelby: It's about precession and concerns only the Harrier. Stators are necessary because of the flow direction after the preceding stage, but only when they have the same direction of movement.

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What are these teardrop shaped thingamabobs on the bottom of the Panavia Tornado?
What are these two teardrop shaped thingamabobs on the bottom of the Panavia tornado?

They could be hardpoints but I haven't found any photos where anything would have been mounted there. They don't look like hatches. And they don't seem to be any kind of vortex generators either. What purpose do they serve?
Click to view full-sized image:

Here's a few closer images:

Maybe chaff launchers?

They're not symmetrically aligned, so probably not hardpoints? (That's a guess -- it just seems like if you're going to be mounting things on the plane, even close to the centerline, you'd probably want them symmetric.)

So, I did find something online that says they're "engine oil" -- I'm not sure exactly what that means, though maybe it's as simple as "here's where you plug in to change the oil"? The page I found it from looks like an official document, though, and says "NATO RESTRICTED" on every page. It's easily found on google, but I don't know if I should be posting it here. Anyone have guidance on that?

@yshavit Big Red has google too ;-P I'm sure it's fine to post.

They appear to be related to changing the oil.
The Tornado's flight manual is available online. Page 448 shows what seem to be the features you're talking about:

The features are the ones marked as "12." The previous page has a legend describing them simply as "engine oil."
This probably also explains why they're not placed symmetrically. It seems the engine oil access is off to the side, so each teardrop is a bit to the right of the engine's center line (that's most visible in the second picture).

Didn't even occur to me that the flight manual would be available, much other interesting information there as well. Thanks for sharing!

@Steve If NATO comes after me I hope the 45 InternetPoints will be worth it... ;)

From my remembered knowledge as a Tonka Av Tech  they are the fairings for the RB199s Oil breathers to allow the oil system to vent oil if the sump is over full or over pressured. A Tonka sootie mate may well tell me Im wrong.

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What are the "panels" on the wingtips of the Panavia Tornado?
I am wondering about the purpose of the panel marked with the number 156 in this picture:

The same black panel can be seen on the wingtips here:

And also here:

What purpose does this panel serve? It could be just a construction technique, but in that case why would they cut a hole in the larger panel just to cover it up with the smaller black one?

From another cutaway drawing:

57: Wing tip antenna

Found here, but the original source is unknown.

Thanks! In all the cutaway pictures I could find this part was missing or the text was unreadable, somehows I missed this one.
Any idea what the antenna would be used for?

Maybe somebody around here will know exactly, but the fact that they are only visible on some aircraft makes me think they are part of an optional or retrofitted communication link.

I worked on GR1s and they weren't fitted to those.


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Could the Boeing 747's 'hump' be improved aerodynamically?
Couldn't the Boeing 747's aerodynamics be improved by extending the hump further back, or sloping it back more gently? Wouldn't this make it more aerodynamic and therefore fuel efficient and faster?

"by extending the hump further back": It would increase weight, but what makes you think it would decrease drag. The wings don't have a rounded rectangle shape...

If I can find it, somewhere on av.se it was explained that the hump actually decreased drag and increased overall efficiency

The 747-300/400 has a longer hump than the original 100/200. Surprisingly, this longer hump had lower aerodynamic drag than the 200 hump: at higher mach numbers compressibility starts to cause serious drag increase. The speed at which this "drag divergence" occurs depends on the variance of cross section area of the plane. If the cross section of the plane would be a constant value, drag divergence speed would be closest to mach 1.
The longer bump on the 747-300 resulted in a more constant cross section and therefore higher drag divergence number => higher cruise speed.
Could it be made more efficient aerodynamically? Yes, by trying to match the Area Rule as closely as possible.
Source

So is the drag lower with a longer hump but only at certain speeds?

Good question. In this particular case it was, due to cruise speed of a passenger jet being near transsonic. At lower speeds the balance between friction drag and pressure drag would be valid: friction drag is proportional to skin area, pressure drag is a function of the shape. Elongating the hump over the whole plane length would reduce pressure drag, but this would be more than offset by the increase in friction drag. Same goes for going for a gentler slope: at a certain incidence of the back end of the bubble, there won't be a gain in pressure drag anymore, only increased friction drag.

Not just near transonic, it is transonic.

Bill Sweetman had a good article on this subject in Air and Space called How The 747 Got Its Hump. The basis for this was in the development of a large cargo aircraft Boeing designed to compete with Lockheed for the contract which resulted in the Lockheed C-5 airplane. Boeing lost the contract but saw potential in its design as a commercial cargo aircraft and airliner.
The aircraft could be loaded through a nose which would hinge upward, but the hinge point and attachment structure would have to be placed in the spot where the cockpit would have normally gone; the only option was to mount the cockpit high up on the fuselage.  Original designs called for the cockpit to be in a blister type fairing above the nose, but wind tunnel testing revealed the design to be unacceptable and the fwd fuselage was redesigned with an elongated Hump to accommodate the cockpit.
Anyways when the passenger variants of the plane were pitched to the airlines, the Pan-Am bosses became enamored with the idea of using the space in the hump for a passenger lounge, some say out of nostalgia for the lower deck lounges which were installed in the old Boeing 337 StratoCruisers.  As the 747 gained worldwide acceptance, the hump became a signature look for the jet (as I understand it Boeing has a patent on the design for that reason).
It is possible that if the 747 would have had its roots as a pure double deck airliner, the design would have looked more like the A380 does today.

This is a good explanation of the reason for the upper deck but does not seem to address aerodynamics at all, which is what the question is asking for.

In that case the answer is that Boeing 'could' do something, but they won't for the reasons above.


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Is it true that Ryanair flies without fuel reserves?
Is it true that Ryanair flies without any fuel reserves so they get priority to land?
I was told that they declare an emergency for landing because of low fuel levels, that is why they always arrive on time. A pilot friend of a friend told me that, but I am not sure... I used to fly with that company because it was cheap, but now I changed my mind.

Absolute nonsense.  You would be better asking on Sceptics.SE or snopes.com.  Several years ago, they had a few emergencies close together but the regulatory bodies do what they do and ordered them to  review their policies, which they did.  If they did this as a matter of course, they would be closed down.

The FAA would have a field day with them if they declared emergencies for every landing just to get priority. There are reserve requirements and they must land with them or face regulatory action.

This group is what I was looking for!

In all jurisdictions in which Ryanair fly, the minimum amount of fuel they must calculate and carry is mandated by law. It is not for Ryanair to decide. They may carry more, but that's a matter of company ops. EU-OPS covers most of their operations  - "The operator shall establish a fuel policy for the purpose of flight planning and in-flight replanning to ensure that every flight carries sufficient fuel for the planned operation and reserves to cover deviations from the planned operation. The fuel policy and any change to it require prior approval by the competent authority""

No.  It's illegal under federal regulations.  If a RyanAir pilot is doing that knowingly on a common carriage flight or otherwise, the FAA should revoke his pilot certificate.   If it is an unwritten company policy of RyanAir which is habitually employed in their operation, they should have their air carrier certificate revoked.  That is very dangerous.

@CarloFelicione Ryan Air don't operate under the jurisdiction of the FAA, though. I'm not sure why people keep mentioning the FAA

BTW, there are plenty of reasons not to fly Ryanair.  Safety is not one of them. They have only had one hull loss incident, caused by a bird strike and no accidents or fatalities.  Indeed, all major airlines are equally safe from a statistics point of view.

I'm voting to close this question as off-topic. It is indeed about aviation, but is more about hearsay than an actual question. If the site starts to accept these questions, then it may be used as a fact checker for frivolous purpose. You may ask if there are regulations in Europe about fuel reserves, and what happens if an operator doesn't comply with them.

Regarding the discussion of jurisdiction, it depends on which of the two similarly named companies are in question. Ryan Air Services indeed operates under FAA jurisdiction. However, the more likely candidate for this question, Ryanair Ltd is not generally subject to FAA jurisdiction. The former is well known in certain parts of the US, while the latter is well known in much of Europe and worldwide.

You are most likely looking at articles like this one which are not exactly true. There was/is an investigation going on after 3 Ryanair pilots diverted to spain due to weather, an unlikely coincidence at best.
Its a bit dated but you can find the Ryanair fuel policy here (from their own site). Like all airlines they need to carry reserves the FAA regulates this under 121.639 and Im sure there is an equal EASA reg. Now airlines are free to exceed this fuel minimum and I am sure many do as a matter of airline regulations but they are also free to run up against the lower limit of it if they please. If it is beneficial to the bottom line to only carry the minimum legally required by authorities a budget airline may do just that. However there is nothing unsafe about this nor illegal, the plane can still make it to a diversion airport if necessary.
For what its worth (at least here in the US) the FAA is going to realize if an airline is constantly declaring a fuel emergency to land.

In EASA land, a fuel emergency is classified as a "serious incident" and requires an investigation.  So same in Europe.


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How should the pack flow knob be set on an Airbus A320?
On the Airbus A320 there is a pack flow knob and you can select LO(80%), NORM (100%), or HI(120%) according to the number of passengers and ambient conditions. Someone told me you select LO if there is less than 115 passengers HI if it is abnormally hot and NORM if everything is casual.
Do both numbers of passengers and ambient conditions have to be met to select LO,HI, or NORM?
For example, obviously if it is above 100 degrees Fahrenheit you would select HI, but what if it is in the 90s or high 80s? Is there a specific temperature or is it option based?

No. Assuming there are no regulations, you can set it anyway you want, at an expense of fuel consumption.
Do not confuse Ventilation control with Temperature control. Temperature control is another setting. (via Cockpit overhead/FAP)


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Why does an airspeed indicator not simply read TAS?
If the airspeed indicator has inputs regarding air density (static port) and temperature. Why isn't Indicated airspeed from the indicator simply the same as True airspeed

Well, indicated airspeed is the most important speed a pilot needs to know, so it makes sense to have that in the most obvious position.
IAS can be thought of as the pressure that the wings are experiencing - therefore all of the stall, manoeuvring, flap extension, and landing speeds etc are based off indicated airspeed.
True airspeed is really only important for flight planning and navigation. Given that more and more aircraft have GPS nowadays to give the groundspeed, the need for the pilot to know the TAS in flight is further reduced.

Thank you. This is what i thought at first, came to the conclusion that TAS must not that important for flying it self but more for navigation.
But then i read more and as the answer from Martin here below suggests, the IAS will always show the same speed at any height. But we know that for example stall speed and other flight performance is heavily affected by the thin air at for example 20,000 feet. Then you might stall at a speed you deemed safe when at a lower altitude.

@John Your example is incorrect. Let's say a wing stalls at 50k IAS. As long as your IAS remains above 50, you are not going to stall regardless of altitude, temperature, air density, etc. Let's assume a perfect day of 0 wind and a perfect standard lapse rate with standard temperature from ground level to FL200. If you take off at sea level and climb to FL200 with the same IAS the entire way, you're fine. Your ground speed will be increasing the entire time but your wing doesn't care about ground speed, just IAS.

Thank you rmaddy.
Can you explain to me why the IAS would show the same indications at FL20 as at 2,000 feet? I was taught that the airspeed indicator measured how many air molecules were thrusting against the pitot and at very high altitudes that would be a lot less than near the ground. Hence, the indicator would show a lot less IAS as you go higher (not unless you keep up with it by going faster).

@John (please put a @ before a person's username when replying.) Yes, the air at FL200 is less dense than the air at 2000'. You see the same IAS because, relative to the ground, the plane is moving faster so the airspeed indicator sees the same IAS. You seem to be falling into the common confusion of air speed and ground speed. Don't get them mixed up.

@rmaddy if a plane has an for example an 120IAS and is climbing at a fixed GS of 500kts (let's say it keeps that GS constant), at FL200 the IAS would be lower but still at the same GS ?

@John Yes, if the GS is constant, the IAS would decrease as altitude increased, all else being equal. But that's not reality. No one climbs at a constant ground speed. Planes climb based on airspeed. Your fixation with ground speed needs to be resolved. Planes don't care about ground speed except when taxiing.

In its basic form, the airspeed indicator measures pressure differential: between the dynamic pressure (from the pitot tube) and the static pressure (from the pressure port). As it only sees differential, it couldn't possibly tell "true" airspeed. That would require, as you say, factoring air density, which is usually not available to the pneumatic instrument.
From a practical point of view, the aircraft flies in terms of indicated air speed: in the sense that the indicated stall speed will be the same at all heights, where a speedometer showing true airspeed will require the pilot to make a calculation to know what the stall speed (or any other speed related to the movement of the airframe through the air) is. There is a single exception to this, as far as I know, which is the VNE: since the maximum speed is determined not by direct flying characteristics of the wing but by stress and vibration, it depends on true airspeed and not on the indicated one; so, at height, the pilot needs to be aware that the VNE shown in the airspeed indicator is not the real one. At the extreme, we have the "coffin corner", the height where the (true) stall speed coincides with the VNE, and it becomes impossible to fly the aircraft without either stalling it or destroying it.

TAS is as i understand, simply IAS but corrected for air density.  The static port measures air pressure, but what is the difference between air density and air pressure? Are they not both just a measurement of "thickness of air"?

You could have some kind of barometer in the aircraft (in fact, the altimeter is one), and use it for your purpose; but it is not very useful information for the pilot, as I say in my answer. In the usual setup of the speedometer, the static port is used to take the difference with the dynamic pressure. Note that with altitude, both the static and the dynamic pressures change.

@MartinArgerami Vne is indicated, not true.

@MartinArgerami Yatzi! Now i think i understand. It was the line "both the static and the dynamic pressure change". So it's simply a linear relationship between the pitot and static port. Although you get a lot less pressure on the pitot at high altitudes, you also get a lot less pressure in the static port, so it cancels the effect out and the IAS stays the same :)
What im still lacking is the understanding of the "physical" difference between pressure and density. As i see it (which is definately wrong) the two are just a measurement of the same thing.. thickness of air.

@rmaddy: if you believe that VNE is indicated and not true, I really hope for your safety that you are not a pilot.

@MartinArgerami Perhaps the better answer is "it depends". I'm a private pilot and an ultralight pilot. Vne is indicated for the aircraft I've flown. Doing a google search on "is vne indicated or true" supports this but there do seem to be cases where Vne may be tied to true airspeed. So the best answer is "Check the POH of the aircraft in question".

@rmaddy: if you fly low, indeed the only VNE you need to care about is the indicated one. But I personally think that every pilot should understand what the Coffin Corner is; and it's precisely the fact that the speed that will destroy the aircraft is tied with the real one, and not the indicated one.

@John Pressure and density are interdependent, but are two seperate things. One is a property of a substance and the other is a form of energy. Imagine a sealed, rigid container with air inside. There is a given mass of air molecules in a given volume. Density is mass/volume. If you heat the air, the mass and volume will not change, so density stays the same. But since you added energy, the pressure will increase. Pressure is caused by molecules banging into each other and the more heat you add, the faster they move and the more they bang into each other.

Say the container is now flexible, like a balloon. When you heat the air, pressure will increase, but since the container can now expand, the density will decrease. So density, pressure, temperature and the makeup of the air (humidity, etc) are all separate things that are interdependent on each other. The pressure of air is created by the weight of the column of air above it. If you increase the temperature of a parcel of air the molecules will spread out lowering density at the same pressure.

@John, Density is just the "thickness" of the air. Put a fixed quality of air into a closed container. it would be just as thick (dense) at 200 degrees below zero as it would be at 15,000 degrees, but at 15,000 degrees the molecules are moving around much, much faster and when they hit something and bounce off they exert a much greater force. ergo, higher pressure.


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On modern fighter aircraft, why do most carry a rotary cannon when they tend to have little use against aircraft?
I understand that in multirole or strike aircraft the cannon has a CAS use, but on a purely fighter aircraft, does the benefit of a fallback weapon outweigh the handicap of added weight?

I think this question is highly opinion-based and should be closed.
But here are my $0.02
Historically there were attempts to remove cannons but turns out too early: Missiles are not that good. The benefit of fallback weapon indeed outweight the weight.

MIG21 VS RF-4C
One instance of combat during Project Dark Gene was an engagement on
  November 28, 1973 between an RF-4C aircraft piloted by IIAF Major
  Shokouhnia and backseater USAF Colonel John Saunders and a Soviet
  MiG-21 flown by Captain Gennadii N. Eliseev. The Soviet pilot fired
  two Vympel K-13 missiles at the Iranian aircraft, failing to destroy
  it. He was ordered from ground control to press his attack at any
  cost, and with his cannon jammed after the first shot,[8] he continued
  by ramming into the Iranian aircraft and losing his life in the
  process. He struck the RF-4C's tail assembly with his wing and then
  flew into high ground. It was the first deliberate jet-to-jet ramming
  by a Soviet aircraft during an interception, a practice common in the
  propeller age of World War II. Eliseev was posthumously awarded as a
  Hero of the Soviet Union.[3][9] The crew of the RF-4C aircraft were
  captured by Soviet ground forces and released after 16 days.2[10]

US military reached the same conclusion around the same time as well, to add cannons to F-4 Phantom II.
Maybe now is the time to rethink this question again after 40+ years advancement of AA missiles. But if you ask the pilot, for the same weight, do they want fuel, gun+ammo, or nothing? A significant portion, if not the majority, would pick the gun I guess.
A lot of fighter pilots, or even astronauts, carry pistols. How useful is that?

It's not just the reliability of missiles that's a reason for guns, it's also their limited number. Once your precious 2-4 missiles (the normal warload of most aircraft) is exhausted, you're now useless except as a pair of eyes. The gun extends that to several more engagements. And do remember that it's normal practice to fire missiles in pairs to make avoiding the attack harder.

If you seriously think it should be closed, you're saying it shouldn't be answered, and you say because POB which tends for VLQ answers from low-rep users . So why did you answer it at all?

@Nij For fun, obviously.

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Why wasn't there a belly gunner on the B-25 Mitchell?
The B-25 Mitchell doesn't have a belly gunner, which would seem quite dangerous. Why didn't it have one?

The production B-25B, B-25C, B-25D, and some B-25G models did have retractable remote control belly turrets. They were often removed in the field because they were ineffective and disliked by the crews.
The lower turret was officially deleted in the middle of the B-25G production run and continued with the B-25H and B-25J production.
More info on the B-25 here: B-25 History.org

My research has shown that the turret was operated through a panaflex prism periscope that caused such intense vertigo and nausea in its' user that is was rarely used and often removed.  In the Pacific Southwest, the turrets were immediately removed and replaced with fuel tanks to increase range and also because monsoon rains turned airfields into mud which covered the gunsight on takeoff rendering the turret useless.

Welcome to Aviation.SE! You could improve your answer by citing the sources of your research.

doolittle had them removed for his raid on tokyo because they interfered with carrier takeoffs and landings

Welcome to Aviation.SE! Sounds like a plausible answer, but could you provide a reference?

My guess is that that conventional fighter aircraft couldn't attack from below (because they have forward-firing guns and can't fly vertically upwards) -- so belly-mounted guns are the only guns you don't need.
The Germans introduced upward-firing guns (Schräge Musik) so that they could attack from below (to take advantage of this undefended approach), but these didn't come into service until late in 1943, and (being on nightfighters) weren't discovered by the Allies for the next year or more.

I don't think there's any reason why "conventional" fighter couldn't attack from below. Why do you say fighters can't fly upwards? Flying straight up is not necessary - even shallow upward angle would do. Or am I missing something?

Flying upward slows you down -- for whatever reason, didn't they tend to dive from on high or something? And a shallow upward angle could be counter-attacked by other gunners in the bomber (e.g. side or rear).

All tactics were used. Against bombers with limited forward facing defenses (like the early B-17 models) forward head-on attacks were taught, but against bombers without belly gunners, it was no issue to burn a little energy with an upward attack for fighters like the Focke Wulf 190. Generally pilots were taught to take advantage of any blind spots, and for fast fighters, exploiting these weaknesses weren't an issue. That's why escort fighters were introduced.


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How do airplane "life jackets" compare to marine life jackets?
I have recently taken up boating in addition to being a pilot, and that experience has given me a better understanding of marine life jackets, also called "personal floatation devices (PFDs)". For boats there are four different types of life jackets, Type I, II, III, and IV, with Type I being the best.
On my boat I typically wear an offshore inflatable life jacket which qualifies as a Type II and as backup I usually have a Type I sitting or hanging in the cabin where it can be easily reached. These are big, strong, bulky things. By comparison the "life jackets" I see stewardesses wear when they do the monotonous lecture at the beginning of the flight seem to be extremely small and flimsy. How do these things compare to marine life jackets?

Marine inflatable on the left, airline inflatable on the right.

Source - Been a pilot for years. Sailboat liveaboard for 1 year.
Aviation life jackets are reliable, but the overall intent is for a rescue operation to occur within hours.  One piece of advice is to stay away from debris.
Marine life jackets are designed to provide flotation for a longer amount of time.  They are also more durable due to the material in which they are constructed from.  Marine life jackets are also designed for multiple uses.


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Can a fixed pitch propeller be designed to act as both a propeller and a generator?
The angle of attack (AoA) plays a major role in designing a propeller for specific uses, such as if it is to be designed for aircraft propulsion the AoA must be high, while the AoA must be low if it will be utilized for generation. Can a single AoA be chosen for a fixed propeller so it can be utilized for both generation and propulsion uses?

What sort of generator?

As the airflow velocity plays a critical role in angles of attack, it's important to understand at which wind velocity the generator blades will be used. Without further indication from you, it can only be assumed you want to use the propeller like a RAT, meaning in a glide velocity range, and you have no constraint on torque or rpm.

There are two close votes, but zero feedback on the reason.  Please offer constructive criticism if you think this should be closed.

@MichaelHall without further constraints like "at the same airspeed" or "same rpm" etc, the answer is simply "yes" with no additional useful information to provide

@Abdullah, I agree, but this wouldn’t be the first question with a very basic answer.  Yet there seems to be no shortage of people willing to draft lengthy explanations to such simple inquiries.  Don’t like the question?  Offer a suggestion to improve it, or downvote it for not being useful.  This isn’t a great question, but why close it?

Maybe we need a forum for any and all ideas.  Aren't we supposed to vote on these things?  Yes, a prop could not only be used as a generator (to recover energy in a descent), it could also function as a spinning flap, to add drag.  Variable pitch would be greatly helpful.  Pet project:  Ford Trimotor with an electric nose engine ...

Long story short: Indeed a propeller can be used in both ways - as propeller or turbine.
To achieve good performance the blade profile normally deflects the air flow. And there is the main difference: for propeller and turbine this deflection is opposite. In the design condition the air follows the propeller profile very well, if you use the same as a turbine the flow will stall and the aerodynamic performance is quite bad. That means either you have a good propeller and a bad turbine or vice versa.
As a compromise you can minimize this effect by using a non deflecting profile and a small stagger angle of the blade. As a result the specific power of propeller resp. turbine will be relatively small.
Coming back to your initial question the aoa resp. stagger angle should be small enough so that no stall occurs for turbine operation. The onset of stall is depending on the actual profile used and the flow inlet conditions. A lot of stuff you need to know to get a specific angle ;)
Cheers

"the blade profile normally deflects the air flow. And there is the main difference: for propeller and turbine this deflection is opposite". That doesn't seem correct. For the same relative flow and airfoil, the deflection is the same.

@Rednose won't you guide our sleigh in the right direction by editing "propeller and turbine deflection is opposite".  Remember, the airstream drives the prop in the event of engine failure. (windmilling).

"the propeller deflection drives the airstream and the airstream drives the turbine blades" i get it, but what happens to the direction of rotation of propeller and the direction of torque in both cases? are they same or reverse @RobertDiGiovanni please answer!!!

@MohammedSiddique to the delight of one restarting their engine, the direction of rotation is the same.  This is because the "push" from the airstream moves the blade the same way as the torque of the (live) engine tries to "pull".  Visualize with a diagram, it works regardless of direction of engine rotation.  Focus on the blade angle and how the airstream would move it if the engine stopped running.

@RobertDiGiovanni thank you for your explanation sir.

Hey guys, sry I was off for Xmas holidays. I am going to add some sketches for better explanation at the weekend.


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Does Airbus buy Boeings and vice versa?
I read somewhere that Airbus developed the A350 in response to the B787. In general, how much do Boeing and Airbus know about each other's aircraft? And how do they learn about the detailed designs of those aircraft? Do they buy each other's planes and study them piece by piece?

Anecdotal: When I visited the boeing factory in Seattle for a tour, all cellphones/cameras had to be left in a locker in the lobby. They were serious about industrial espionage!

Airbus used to own Super Guppies to transport aircraft parts around Europe.

“In response to” does not suggest copying anything from the actual design. Just designing something for the same market segment. A350 is actually a much more conservative design than B787.

I was not necessarily implying copying. But there's a large spectrum between plagiarism and being inspired. Btw, this has happened in other industries (e.g. automobile). Also, with Airbus manufacturing in China as well, it's been discussed whether such a thing could happen to Airbus from the Chinese.

When a manufacturer is making an aircraft to compete with another manufacturer's products, they have the public marketing material to go off of, as well as what their customers tell them the other aircraft can do, and what they are looking to buy. The general performance capabilities and costs are what is most important at the high level of making a competing product.
More in-depth study does help for detailed information, and there are multiple ways that this can happen.
Manufacturers will end up with airplanes from other manufacturers through trade-ins. Just like with car dealers, they may offer to buy older aircraft from a customer as part of the sales deal for new aircraft. Unless they are headed for scrap, these aircraft are generally worth more intact on the used market than dismantling them but it certainly allows closer study.
Manufacturers can also look for parts on the spares market like any other buyer, and get specific parts of interest that way.
Information can also be found in safety bulletins or accident reports if a certain system or component is involved.

Don't forget they have to file public patents on anything truly new.

So in short, @fooot's answer is"no, it doesn't really happen". It's quite surprising to me.

@StefanLafon should not really be surprising considering the actual monetary cost of such aircraft. recouping those expenses would be near-impossible

Could you explain? Developing a new aircraft costs billions of dollars. So buying one aircraft from the competition is peanuts compared to that.


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Third Class medical license, what changed, if anything?
Supposedly, in 2016, Congress passed a new law making it easier to fly without the usual medical examination requirements. However, in looking at one of the rather obtuse "FAQs" on this new law, I don't really see what has changed. You still have to have had a medical exam, so new pilots apparently get no benefit whatsoever, they still have to be examined by a doctor. Not only that pilots still have to pass a medical exam at least every 4 years as far as I could tell. So, after years and years of lobbying for a medical-free third class, is the end result that now pilots only have to have an exam every 4 years instead of every 2? Here is an excerpt for the proposed law as I originally saw it:

Fly with drivers license. Did this not happen?

Short answer: politics killed the "driver's license medical" idea for private pilots, but there are still reasons why BasicMed has significant advantages for some pilots.

The literal answer to your question is "nothing": the third-class medical hasn't changed at all. What you're referring to is an additional option called BasicMed, which you can hold instead of (for some operations) or even in addition to a third-class medical.
I won't get into the details of BasicMed itself (the regulations are in 14 CFR 68), but AOPA has a lot of great information about it. Instead, I'll focus on your "why is this any better than a third-class medical?" question, which seems to be quite a common one right now.
In summary, if you're under 40 and healthy then BasicMed may not seem very interesting. But if you're over 40 and especially if you currently have a Special Issuance then BasicMed could be a much easier and cheaper way to fly.
As you said, as a new pilot you still have to pass a third-class medical once so let's assume that you've done that successfully. It's certainly true that some people aren't happy with that requirement because they were hoping for a "driver's license medical" where a private pilot could fly without any medical certificate, as sport pilots can do. That idea didn't survive the political process, however. If you want to fly without any medical you still have gliders, ultralights or LSAs.
But when it comes time to renew, here are some reasons that you might consider BasicMed (most of this information comes from the AOPA FAQs):

If you're over 40, you need to renew a third-class medical every two years, but you only need to see a doctor every 4 years under BasicMed
Special Issuances still exist under BasicMed but for a more limited number of conditions than for a regular medical. Practically, that means you may be able to avoid expensive tests that the FAA requires for a third-class. This is a huge benefit for some pilots because it can save a lot of time and money (health insurance typically doesn't cover 'unnecessary' tests).
Any state-licensed physician can sign off on BasicMed, but only an AME can sign off an FAA medical. That means in theory it should be easier and cheaper to find a doctor for BasicMed. In practice people's experiences have been mixed on this, but BasicMed is still very new and it's not surprising that some doctors may not be comfortable with it.
Because the BasicMed medical questionnaire is effectively a general physical exam, some people have successfully combined it with an annual checkup paid for by their health insurance

There may be other reasons, but those are some common ones I've read about. If you're a member, the AOPA forums have a lot of discussions about individual experiences.

Please see my edit for excerpt from the law.

@TylerDurden A link to the source for that text would be great, but I assume it's an early proposal or draft. The "driver's license medical" idea was killed by politics; I've linked to a good article about it in my answer.


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How much runway is needed for comfortable operations in a PC-9?
I am familiar with simple single engine props and twins and am thinking of moving up to turboprops like a Soccata TBM 700 or Pilatus PC-9.
How much runway do aircraft like these need to operate comfortably? (think grass strips with trees at the ends). By "comfortably" I am not talking about test pilots in perfect conditions. I am talking an amateur pilot on a windy, rainy day with enough runway that he won't be shaving bird nests out of the trees on takeoff.

Notwithstanding what the manual says, this depends most on the pilot.

If you are considering transitioning to a complex turboprop from simple singles or twins, you might want to consider adding a margin of at least 60-100% to the book figures in order to establish comfortable personal minimums.

Define amateur pilot.  When you ask this question and qualify it by amateur pilot, you are qualifying it with a wide range.  A better qualification might be someone who has completed Flight Safety Simulator Training in the aircraft, or has accomplished an insurance checkout. Even those are tremendously broad. However, if I was giving dual in either plane, I would look for a field with 2X the book figures for starters, with a green pilot, if that helps.

@mongo That seems to be a good guideline. So, basically it seems like 4000 to 4500 feet seems to be the right minimum.

@TylerDurden Yeah, if your aircraft is light and at low density altitude. If you are operating high, hot, and heavy you might want more than that.

TBM quotes the distances for the 700 at

Take-off: 2,133 ft (650 m)
Landing: 2133 ft (650 m)

And Pilatus for the PC-9

Take-off distance over 50 ft obstacle at sea level: 1,300 ft (397 m)
Landing distance over 50 ft obstacle at sea level: 2,255 ft (687 m)

The PC-9 and TBM 700 are very different beasts (did you mean PC-12?)
comfort is about you the pilot and there are some pilots who operate right down to book minimums. Like anything else in aviation the pilot is the ultimate decider and you can operate both of these planes off a 2256 foot strip in clear weather. I'm not sure I would. I would think 3000ft. is enough to comfortably fly either of these planes. The FBO I rent from at KDYL flys King Air 200's out of there all the time and they only have 3000 ft there. For planes like this, considering their common missions I would also look for a strip with a solid IFR approach, sells Jet-A on the field, enough ramp room to fit your plane, and taxiways wide enough for your wing span as well as sufficient runway length. Its also going to take a lot more than "familiarity" with single engine prop planes to move up to a turbo prop.

And as with all takeoff performance, the figures will vary significantly with gross weight and density altitude.

For example, the figures from the TBM 700 PIM show the following distance required to clear a 50' obstacle off of a dry, paved runway: 1591 ft at PA of 0 MSL, ISA, and low weight (5512 lbs); 3182 ft at PA of 4000 MSL, ISA +30°C, and high weight (6579 lbs). That is a very significant increase, and those figures reflect the skills of a test pilot of with no margin for error. Add 7-25% for grass runways.

A good rule of thumb for amateurs: take calculated takeoff and landing then add them together.


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Are the statistics in "The Killing Zone" appropriate and scientifically correct?
I recently ordered the book "The Killing Zone" and am looking forward to reading it. Most of the reviews were positive, but one reader complained that the author made the mistake of using accident frequency counts, rather than accident rates. What is he talking about?
MAIN QUESTION: Are the statistics in “The Killing Zone” appropriate and scientifically correct?
It would seem to me that the natural statistics would be fatal accidents per flight hour. Does the book not use this measure?
I have read elsewhere that the highest accident rates are those involving pilots having between about 200 hours and 800 hours. Does that come from this book?
Interestingly enough, my personal experience with fatal accidents does not correspond with this. Every one of the 8 or so pilots I have known (indirectly in all cases) that have been killed in an accident were high time pilots (> 1500 hours).

You could just read the book and find out :-) It's a really good book, as far as I remember the numbers are based on accidents per hour and the highest accident rates are at 200-500hrs (or so). That 'window' is exactly the killing zone in the title.

If your questions are really, "What is he talking about?" and "Does that come from this book?", then I think you should read the book rather than posting here.

My primary question is whether the statistics in the book are appropriate and scientifically correct. The secondary questions I can eventually answer by reading the book. I have bolded my primary question to make it clear.

If your question is really about the statistics in the books, I'd ask some statisticians about the scientific validity, not a bunch of pilots.

The data can be statistically valid and "scientifically correct" whether the results are presented as absolute accident rates (number per year), rate per X-hours, or rate per-mile. Which metric is "appropriate" depends on exactly what you're trying to study (or alternatively how good you're trying to make yourself look: Accident rates per passenger-mile can look a lot better than per-flight-hour numbers, and per-flight-hour numbers can look a lot better than absolute occurrence rates...)

I did a bit of reading and found the review questioning the book. That review was written by William R. Knecht.
Knecht has provided in that review a link to one of his papers. (Payment is required but the abstract is free to read anyway.)
Instead a bit of searching led me to this paper also by William Knecht which I believe covers similar material.
His argument requires a lot of specialist statistical knowledge to understand, but it is worth a read through, especially the Introduction and the Discussion sections.
There are huge numbers of low-hours pilots, and their accident counts swamp the true pattern of data (my interpretation).  The high-hours pilots have some accidents too, but the data there is noisy due to the relatively low number of these pilots. (Poor data sources are mentioned too.  See the paragraph beginning "We end with an appeal to the FAA and NTSB.") A more sophisticated analysis of this noisy data would seem to suggest that the "Killing zone" is much wider, reaching up to pilots with a few thousand hours, maybe more.
Disclaimer: we would probably need to be professional statisticians to understand every detail of the argument. Everything I've written above is my own interpretation, as a non-specialist.

I have access also to the first paper. I can confirm that it presents basically the same plots/data as the freely available one, made exception for the esposition style adjusted to the different audience.


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Is the Harbin Y-11 a fully indigenous Chinese design?
One book characterized the Harbin Y-11 as a "successor" aircraft to a design based on the Antonov An-2, however, it looks completely different than the An-2, which is a single-engine aircraft. Is the Harbin Y-11 a fully indigenous Chinese design with no significant elements copied from some other aircraft?

It is probably just saying that the Y-11 was the new aircraft to fill the role previously covered by designs like the An-2. Sort of looks like a Twin Otter.

It has a overhead wing with a spar -> many skydiving planes have that as well

While it is possible that the designer lived in a vacuum and took no outside influences into his design, it uses several design elements shown in several other aircraft of that era, designed with the same roles in mind. What do you mean when you say "indigenous"... if you mean what I would, I would say no.

@CGCampbell By an independent design I mean using your own plans and drawings, not reversed drawings based on calipered measurements of existing aircraft. In other words if the measurements are exactly the same as some other aircraft that would be a "copy". If the measurements of the component are unique, then it is an independent design.

Though Harbin Aircraft Manufacturing Corporation (HAMC) said Y-11 is a fully indigenous design, no other direct evidences prove/reject what it says. However, some indirect evidence shall be considered:
1) Except Y11, the only brand new design from HAMC is Z15 in 2008(Coperation with Airbus), other designs from HAMC are either authorised production or modification from existed model.
2) Y11 was designed in the period of the Cultural Revolution. There is insufficient engineers and research process is often influenced by political movement. However, Y-11 started first flight 8 months after the requirement is issued.
Based on the evidences i dont think it is a fully indigenous Chinese design, as they dont have experience for fully indigenous design in such a short period.

pure speculation. China had a lot of smart people, many of them trained abroad. HAMC didn't need to design many aircraft as they produced the Y-11 (and later the Y-12) exclusively for decades. That's how things work in a communist country. Saying the fact they only produced that one model proves they didn't design it is like saying Lada couldn't have designed their car in the 1960s because they never designed another until the mid 1980s...

After some research it appears the Y-11 is an independent design based on the Soviet Antonov An-28 with various significant modifications. The design appears to be a scaled down version of the An-28. Certain elements such as the undercarriage and wing supports are identical.

parallel evolution would account for that easily. The Twin Otter is also very similar in design, and there are others. The configuration is a logical one given the requirements all those aircraft were designed to.


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How Centre of Gravity (CG) is adjusted?
I've gone through the following questions on Stack Exchange and other few questions too:
Why is managing CG (centre of gravity) important?
How are the limits of the center of gravity chart established?
I still do have 2 questions in mind:

How do they adjust the fuel quantity/weight based on the passengers' weight on the plane? (was there any situation where few fat guys have been asked to move to front or back because I never saw any airliner weighing their passengers before boarding)
As the fuel starts burning, the pilot will fill/empty/exchange it from wing to wing or other storage location to balance CG, Will this cause imbalance (moving out of defined tolerances) in CG based on the passengers' weight?

Related question

Related too: How does a commercial airliner measure its weight/mass?

About 1: I usually choose a seat in the last (few) rows of an aircraft, to have a nice view on the wings, control surfaces and engines. Once, a colleague and me were called to the boarding desk and were told that we were reassigned from the last row to  the first for balancing of the Dash 8. The aircraft wasn't full, but we were the only ones getting new seats. We both are not the lightest, which may be good for balancing, but how should they have known this before calling our names?

The main issue is what is referred to as the Weight and Balance of the aircraft. When an aircraft is in flight, its weight and balance needs to be within a controllable range in order to ensure the plane can both recover from a stall condition and rotate on takeoff. This is visualized by the weight and balance "envelope:

As you can see, there are limits where the aircraft is not fit for flight. We must stay inside the drawn lines in the center.
To answer your detailed questions:

On airlines, a "Magic" number was used to approximate all weight for passengers and baggage. This caused at least one  notable accident: Air Midwest Flight 5481. As a result of this accident, policies were changed to get more accurate baggage and passenger weight.
In GA flight, it is not uncommon for pilots to ask for passenger weights. Usually, as a rule, we add a bit to that in case people are embarrassed (10%). It is very common for pilots to assign seating on smaller aircraft with heavier people sitting in a location where the distance to the datum is smallest (near the front).
Weight and balance can be moved out of limits by fuel burn. In fact, a few questions on the private pilot written exam deal with this exact scenario. Most planes have tanks near the CG, so they don't experience problems with fuel burn. In planes with an aft belly tank, the CG can move forward significantly and out of limits. This would cause the aircraft to be unable to pitch up as desired. Swept wing jets that use horizontal stabilizer tanks in the tail of the aircraft have computers and pumps to move the fuel as desired to keep their CG in the desired place.
Pilots of smaller/less traditional airplanes need to be conscious of this, and plan accordingly. Especially those with a-typical fuel tank positions. Pilots of larger commercial style aircraft are less concerned with the exact position of 1 or 2 large people or heavier bags because their aircraft has a much bigger capacity and less "sensitive" envelope. They still are affected by the physics, though, just like everyone else.

In the majority of airplanes fuel is stored in the wings and tanks which are positioned at or near the center of gravity at empty. This limits the forward-back moment arm of the fuel so as fuel is used up the center of gravity does not shift forward or back that much. Balancing fuel weight is therefore more of a question of lateral CG, a pilot (or automatic onboard system) can pump fuel or switch tanks to balance fuel load. The seating configuration of passengers does not have much impact on lateral CG as their moment arm is very small.
In light aircraft passenger weight and position is a big concern, however passengers are actually one of the smallest weight components on a commercial airliner compared to the plane itself, fuel and cargo. You won't see overweight people being asked to move to balance a commercial aircraft. Seating systems deal with numbers and are designed to spread the load evenly across CG. Even if all the passengers of a half-empty commercial jet sat at the back there would still be enough trimming power to keep the airplane in balance.

As said in my comment to the question: Being asked to move from the last to the first row was exactly what has happened to me once on a Dash 8.

In some newer planes i.e.B-787, A-380 etc., the CG is adjusted after loading the cargo.  The landing gear have solid state scales built in and in the extreme tailcone is a jackscrew with a very heavy weight attached, usually made of dense depleted uranium.  This can be moved laterally +/_ several feet to adjust the CG.

usually made of dense depleted uranium I seriosly doubt it and won't believe you unless source is provided.

You should add links and references to enhance you answer.

@Federico: According to Wikipedia at least, DU is sometimes used in civilian aircraft.


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Why are winches not used for towing commercial planes up?
In this video a glider is pulled up by a winch on the ground, like this:

(Source)
I would say you could apply the same principles with bigger planes, possibly even using sustainable energy sources.
The plane would not use its engines at all to get to cruising attitude (unlike a suggested catapult in a previous post), so this part of the flight would be "for free".
Using a back of the envelope type of calculation with 5% efficiency it takes 17000 litres of kerosine to lift a 747 to 10km (which agrees with this paper).
However, this would however cost at least (calculated below) 25km of towing line(s). If we use a UHMWPE fiber a 400g/m and 1.2 cm radius cable would be able to support 2x the 400kN thrust maximum output of a typical airliner.
Sure there are a lot of hurdles to overcome, but since some airports in cities cannot expand because of noise problems this seems like one solution.
Logistically this seems like a large extra challenge for the crew on the ground, but would this be possible?

Comments are not for extended discussion; this conversation has been moved to chat.

You mostly answered your own question in your bounty post :: I'm still not convinced this is impossible, while not useful for less densely populated areas this could be useful for densely populated countries :: It's not impossible; anything can be done if someone has the will. But cable-launching heavy aircraft has very little usefulness, no matter the population. Why do you think it could be useful in densely populated areas?

"I'm still not convinced this is impossible." Then it's an engineering question. How thick of a steel cable would we need to slingshot a 747 to cruising altitude? According to one of the answers (uncited! @mins), it's : too big.

We calculated in the comments it could be done with a UHMWPE wire, with a factor 2 safety ratio the wire would have to be 1.2cm radius and 400g/m

I believe this has to be done against the wind. If that's the case, it's the wind that raises the glider, and the cable just anchors it. For most airplanes there's simply not fast enough wind to lift it.

Gotta say, *boggle* at the bounty. One of the answers is three screenfuls of my browser; another is four. Both posted long before the bounty. How much detail do you want?

"The plane would not use its engines at all to get to cruising attitude".  And then 10 km up, the pilot would begin the checks and start the engines and they would work perfectly every time.  There's a a very good reason why planes run their engines for several minutes before takeoff.

I've never done a winch launch in a glider, but I have done a few hundred aerotow launches. I never really viewed winch launch as a viable option. Best you're going to get is 1200 feet or so - perfectly fine to hunt around for a thermal in ideal conditions (which I've never experienced in 10 years of flying them). So, assuming your passenger liner only needs to go to 1200 feet or so and the passengers don't mind catching a suitable thermal once every 10 or so years, it seems perfectly reasonable.

Airports not expanding in cities isn’t a noise problem, people willingly pay for homes right under flight paths due to price.  But have you considered the amount of real estate you’d need to buy up and clear out to allow miles and miles of cable to safely free fall from 35,000’ without striking cars and houses?!  If you think extending a runway at a busy city airport is a difficult project…

Even a launch to say 3000m ( 10000 feet ) would leave 3000m of cable in the sky. Where is that lot going to fall?

@nimbusgb I imagine not allowing it to fall, keep on winching(fast). Maybe a small parachute on the end and never let it touch the ground.

5000 to 10000 m of cable capable of withstanding the force to winch launch a 250 tonne aircraft is going to need a 'chute that weighs a tonne itself!

If you ever get the chance, do try a winch start in a glider. It is, to say the least, an interesting experience. I do not believe it would be accepted by the general public.
So, that is one more reason, it would scare the paying passengers.
--- addition after comment
There are several scary parts to a winch start that. Passengers will of course eventually get used to it, but still.
.1 The least scary part is actually the accelaration. There will be a bit of that, but probably not more than from a normal start.
.2 One scary part will be the angle of attack. In a glider the nose points well inte the air in the start. This is in order to gain enough height before release. With a longer tow rope you can either get higher or a less steep ascent (how long tow rope would you have?  )
.3 And on release from the tow rope there will be a noticeable change which probably could scare as well. Hopefully the engines are running full speed at that point.

I have not tried it myself... but surely, in a glider it would feel a lot rougher than in a jetplane due to the lower mass?

@leftaroundabout But surely, in a jet it would feel a lot rougher than in a glider due to the required higher acceleration. Jet has a much higher desired speed and you have a limited distance, so you will need a lot more "g". Mass doesn't even appear in this equation.

@kubanczyk that's not the point. A glider can accelerated so quickly (and in particularly, so roughly, which I presume refers to a pretty sudden “kick in the back” at the beginning), because it's so light. That's good of course because it allows very quick takeoff and therefore much height to be gained without excessively long rope. For a fully fueled jet though, that much acceleration would simply not be feasible, you'd have to make do with less g spread over a longer way. Then again, that would make the rope length totally impractical, which is really the main reason this isn't done.

I'd say you'd need about the same acceleration as you get now. More is better (within reason) because of the higher altitude you can reach with a glider and because you want the glider to fly and be controllable as soon as possible.

Even more interesting to the general public would be the recovery action in the event of a rope break, especially considering a glider only has to sustain that horribly negative g for a couple of seconds before it reaches an appropriate recovery attitude.

Why? This answer doesn't really explain anything except to note it is an interesting experience.  Why would it scare anyone? Isn't sitting in a metal tube with engines strapped to wings, accelerating toward grass or water in the hopes the plane lifts off not also an interesting experience?

What's the answer here to, "would this be possible?" What exactly happens when I try to winch start in a glider, using a motor capable of providing the necessary torque and a cable capable of withstanding it? I'm guessing it goes... up.

A rough kick in the back? Nope, a good winch driver brings the power on quickly but under control. Most will be using an automatic torque converter tye transmission so it's not a 'kick in the back'. We are trying to launch the glider not rip the hook out of the fuselage.

I won't prove it's not possible, but I'd would be surprised it is. I'll develop the reasons why and provide the physical framework you can use to compute a solution to have, with some effort, a final answer.

The glider case cannot be scaled up to an airliner
It works with a glider for two reasons:

The height to be reached before the glider is able to gain altitude by itself is limited: 200 m or so. To be compared with the 10 km the airliners fly (as you want to reach the cruise altitude without the engines).
A glider has a lift-to-drag ratio of 30, 60 for best gliders, a B747 has a L/D ratio of 17.

These two differences have huge interlocked consequences:

When you raise a cable, the top segment of this cable must be able to resist the whole weight of the cable, plus the drag created, plus the force that the cable needs to transmit in order to move the aircraft forward. Each newton or kg added to the wire increases the required section, hence the weight, hence the section, etc.
If the L/D ratio is lower, then the aircraft will create more drag when it will create lift to lift the cable, hence the force created by the winch must be larger to move the aircraft forward, hence the section of the cable must be larger, hence the first problem is aggravated.

Description of the problem

The cable curve will be a catenary, actual calculations by an engineer should prove the solution isn't possible with the material we have today (including a good candidate, Dyneema used by tug boats).
Kite analogy
Those interested may also try the Kite Modeler Java applet from Nasa, as in this configuration the airplane is mostly a kite:

Source
Additional problems to solve:

How to manage a safe corridor long of something like tens of km?
How to reset the winch for the next launch?
Effect of the wind on the cable, and corresponding jolts on the aircraft.
Cable elasticity effect on the aircraft.
How to continue operations when static electricity starts to accumulate in air, and you don't want to transform the system into a lightning rod?

"L/D ratio of 17 cannot climb efficiently more than 1 km while moving forward by 17 km." That ratio does not tell you anything about the climb rate. Look at the 15% used here currently which is a lot more than 1km/17km

Static isn't a problem: just make sure the entire airplane-cable-winch system is electrically bonded and grounded, and any static will dissipate as fast as it builds up.

The altitude you can reach this way is hardly limited. My personal best is a bit over 700m (on a relatively long field with a strong headwind). Also, I do not buy your argument that the rate of climb is limited by the L/D ratio. A plan can climb much steeper then its L/D ratio: a glider (L/D > 30) can climb to about 1/3rd to 1/2 the runway length, depending on wind and operator skill of the pilot and winch operator.

Nice math, but wrong entry data. L/D is for plane as closed system, expending stored potential energy to compensate drag loses. Winch is adding energy to the system. For a glider - all of it.

I don't see how the L/D of 17 converts to a 170 km horizontally for a 10 km cruise altitude. For higher L/D (e.g. 30 for a glider) would it mean that more horizontal distance is required? What if we have an extremely efficient aircraft with L/D of 10 000. Would a meter climb by cable towing than require 10 km of horizontal distance?  That doesn't make sense. I'd expect it to climb steeper

@DeltaLima: You're right, this coefficient is not used correctly, let me rethink that, but that won't change the overall result anyway.

@mins I agree it doesn't change the outcome. Also don't forget consider the speed of the aircraft. In a glider start the speeds are low, but a passenger jet would required approximately 150 knots. The cable is almost perpendicular to the flight path (say 30 degrees off), which means that the cable is moving sideways through the air at 150 * cos(30)  = 130 knots. That is going to give a significant drag on the cable alone.

@DeltaLima: Correct, in addition the aircraft might not be able to withstand the wing load required, as the aircraft would, for a good part of the climb, "fly" (or actually drag) with a huge angle of attack in order to maintain the cable tension.

@mins My guts tell me that with attack angle approaching 90 deg and winch pull approaching infinity you should get climb path approaching vertical (at airspeed approaching infinity). So IMHO the only limit on shortening the runway is material strength. My limited experience with kites show that forces on the cable (and wings) are huge, as eventually it may exert force at least as large as it's max lift when it overflies the winch. OFC the trick would be to release it before that moment, but if you want to pump the most energy into the plane, you need to pull as long as possible.

@Agent_L: To put the situation into reasonable perspective, winch-launched gliders typically climb at around 1:1. Although it's now prohibited in competition, it was once fairly common for radio controlled gliders to do what was called "kiting". They'd use the winch to launch about as high as possible. Assuming a headwind they'd keep the glider on the winch, and let it pull the cable back out as far as possible. Then re-start the winch to climb higher (and possibly repeat). At the end, do a small dive to gain still more energy that you can use to climb still higher after release.

"this solution isn't possible with the material we have today." There it is, the answer to can we build this. That quote should be in bold or a header to the whole thing, or at least a giant big NO. Why are winches not used for towing commercial planes up? Because we can't scale that to the size necessary.

Scientists expect that the (rope) material to build an elevator in to space will be available soon. That's 35000 km straight up. If that should be possible soon, then surely we can pull a little airplane up 10 km with a winch.

I did a quick check on an online catenary calculator, but it seems that it should not be much of a problem. I assumed a cable pull of 500 KN, a winch to plane ground distance of 25 km and a rope weight of 1.5 kg/m (Dyneema SK75). https://www.spaceagecontrol.com/calccabm.htm?F=500000&a=50000&q=1.5&g=9.81&Submit+Button=Calculate

@rickboender: If you understand the concept of space elevator, then you surely know the principle for tightening the "rope" is not based on limited lift but on quasi-unlimited force from Earth centrifugal acceleration.

@rickboender, thanks! The catenary indeed does not seem much like a problem. Not for the 50km line of 1.5 kg/m line and certainly not for the 25km 400g/m line I was assuming. mins, I don't think this was the point. As a physisict I would argue that the breaking length of the "rope" material in a space elevator is one of the mayor challenges. The point rickboender is trying to make is that in respect to the space elevator this problem (which I might agree is non-trivial) could be solved overnight using current materials and methods.

@Ruud3.1415: "50km line of 1.5 kg/m". Perhaps it's possible but let's still remember a dynamic mountaineering rope, to support only 80 kg with the safety margin for acceleration in case of fall (the aircraft will also need this due to unstable wind/tension) already weights 60 g/m. So according to these standards and with the same material, 1.5 kg/m is safe for 2 tons. The comparison stops here, but we need to dig into the details, like the safety calculations, including the required elastic range, to understand where is the balance. I'd be happy to see Rick solution with figures.

@mins, let's look at the numbers here (don't ever try to fall in dyneema rope, you'll break your back). The specific strength of UHMWPE is 2514 kN m/kg so 1.5kg/m would translate to  7.5 times the required for maximum thrust. Dyneema or UHMWPE is one of the stiffest materials out the NOT dynamic, I fail to see why you would need dynamic rope.

Let's go in the [chat] room to continue the discussion. (This one)

(I hesitated to write this answer because I couldn't find official sources, but in the end I decided to do it; the source is that I flew gliders, instructed, and dealt with their maintenance for many years)
Something not mentioned in the other answers is the stress on the airframe. A glider that is winched regularly has a reduced lifespan. Like I said above, I don't have concrete sources, but I do remember being involved in the extension of the end-of-life of a Blanik, and to determine how many hours were allowed; the manufacturer considered regular winch launches on the same level as regular aerobatics (that is, as something that substantially reduced the life span of the airframe).

'cause you'll break the plane +1

In addition to the other excellent answers, two more strikes against cable launch in the commercial aviation industry:

Need control and power for emergencies
Additional point of failure and loss of runway
Runway damage

First, an aircraft will always have full power upon launch whether cable launched or not.  This is to give the widest possible safety margin in case a problem comes up.
Second, a cable launch mechanism adds an additional point of failure.  Not only could we end up not being able to launch due to a cable launch mechanism problem, but it could prevent that runway from being used for takeoff or landing depending on the problem.
Lastly, the cable and its attachments must be necessarily heavy since they will be slamming into the ground or perhaps a plate designed to absorb such impact when released. However not all releases will be ideal, and if the cable or its attachment hits the runway it will be compromised, potentially halting all traffic on that runway for many hours.
All these things can be handled, but are additional factors that would prevent an airline and airport from adopting such a system.
However, there's an additional positive offset:
Takeoff uses a significant amount of fuel.  If the takeoff can be energized from a ground power source, particularly where electricity is cheap, you may be able to offset some fuel and its associated costs for each flight, which could result in a significant savings over industry flights.
This would also reduce (or at least relocate) carbon emissions.

an aircraft will always have full power upon launch whether cable launched or not - Not quite accurate, especially in commercial aircraft. Takeoffs are quite often done at less than full power for various reasons.

As a glider pilot who mainly uses winch launching and also a professional engineer I can see both sides of the issue.
You can solve the launch issues by throwing engineers and money at the problem but there is a glaring issue that does not seem to have been covered: cable breaks.
Despite having over engineered cables and linkages, cable breaks do occur, I have had a couple and they can be sphincter-clenching moments but we train for them and we survive. We survive them mostly because we are flying gliders, aircraft that are specifically designed to be flown without power.
A break at low level and I just land beyond the winch, this would need a massively long runway with the winch partway up, a mid level break and I turn and do a cross-field landing which is easy as my airfield is very wide but a commercial airport has fairly narrow runways. A higher level break and do a quick circuit of the field and land normally.
An airliner is not a glider and so the land ahead is possible but the circuit and land normally would not be possible due to the glide characteristics of an airliner. A cross field landing would require a square runway which is massively expensive.
...and then you have the problem after a successful cable release of several tons of cable dropping to the ground. In gliding this is done by means of a parachute under tension from the winch. In the event of a winch failure at cable release then you need to have a clear area all around the winch the same diameter as the release height in case the cable drop onto something or someone. Unlike gliding this cable will be very very heavy.

any idea how much time it would take to spin up the engines as a backup-strategy?

About the falling wire when it would break, I calculated that the terminal velocity of the 1.2cm radius 400g/m wire I mentioned above (2x strength of full thrust engines) would be around 86km/h or 53 mph. I think getting hit by this would not be likely to seriously injure someone and the wire could easily be made even thicker to reduce terminal velocity.

As a glider pilot engines are voodoo to me but I would guess that the engines would be running and ready but if you were low enough to the ground and had a break the time between full power and an uncontrolled stall would be uncomfortably close.

@Ruud3.1415. That is most likely correct for a linear fall but there can be a 'bullwhip' effect when the wire drops and the end flips out at a very high speed. In addition you have a weight on the end for the attachment gear (assuming the wire falls intact) which can do a lot of damage.
The wire I launch on for a 500kg glider is about 1cm diameter. I suspect that for safety an airliner version would be grossly over specified and likely to have protective sleeving to prevent the abrasion from being hauled along the runway at speed. I take off on a grass field and the wire still gets worn.

Just as an extra, gliders do not fly as they are launched, they 'kite'. On a cable break your glider instantly stalls and you have to take a (very long) few seconds letting the nose drop and then dive to get airflow over the wings so that the control surfaces start to work. The flight characteristics of the glider allows for a quick orientation change and control recovery. A large aircraft would respond much slower and a low level break would not allow enough time for it to re-orient and land even with engines spinning up to speed.

Cables are heavy, let's do railguns
Essentially gliders are light and only need to be pulled to a modest height, an airliner is vast and you could only hope to save a fraction of the takeoff energy. The plane couldn't be made smaller unless it only took off from such places. And the cable to pull an airliner would be incredibly heavy, and you'd only get 3km or so (the length of the runway) to use unless you think airports always have space beyond the runways.
A railgun, though, would be more fun.
How fast can we go in 3km?
Imagine we accelerate at 1g (total force on the passengers is then 1.4g) for 3km (reasonable international airport runway). v^2=u^2+2as -> final v=240 m/s or 540 mph which is pretty close to cruising speed. So we don't need even a 3km runway for this, or we could accelerate more gently.
There is a hitch - we're still on the ground. So in fact the best thing is to assist the aircraft to a takeoff velocity (v2) and let the rest of the takeoff proceed as normal.
If that's so clever, why isn't it being done?
It is in development. The EM Assisted Launch System for the US Navy, to replace catapult launch from carriers.
But, as this answer explains, the proportion of an airliner's energy used to take off is a small fraction of the total energy used, so you are likely to lose any benefits in additional weight from implementing such a thing.

This idea is similar to this post. While it could reduce the length of the runway needed, the energy an noise savings would not be fairly significant.

Good point, have updated.

I don't think your answered the question which includes "The plane would not use its engines at all to get to cruising attitude".

@mins: Well, the basic answer to the whole requirement is No, so it seemed better to elaborate on the approximate concept.

A fairly obvious point that seems to have been missed in the other answers: how fast can you cycle the winch?  A busy airport might have planes departing every couple of minutes.  To get to cruise altitude, your winch has to reel in a considerable length of cable.  This then has to fall to the ground after the aircraft releases it, which takes a minute or two at best, more if it has a parachute.  Then a ground crew has to grab the end of the cable, reel it back to the departure end of the runway (which requires a vehicle), and attach it to the next plane.  Say you can do all that in a really optimistic 10 minutes: that means you've cut your airport's peak departure capacity to 20%.
Then you have the cost of all this infrastructure, plus you have to duplicate it so you can take off in either direction.  And that's not even getting into what you might do for intersecting runways (or even parallel ones), or how the cables might affect landing traffic...

Clearly you would have more than one winch-reel - deploy No.2 while you are launching No.1, recover No.1 while launching No.3 ...

@MikeBrockington, but how many would you need at a busy airport like LAX, DFW, etc?  How long would it take to get each airplane to cruise altitude?  Clearly you would need more than one, clearly so many as to render the idea pretty far fetched, don’t you think?

Assuming that you still have to deal with separation due to wake turbulence, then maybe not all that many. Depends whether/what setup you have for paying out the winch cable, and in particular, whether that can be done while other planes are taking off. More of an engineering problem in my mind, than a full-blown impossibility.

I believe it is a good idea, and that it will happen eventually, but not anytime soon though. With the airplanes we have now it is simply not worth it economically. We say that bringing extra fuel on a plane is very expensive. It is expensive because, if you bring extra fuel to extend your range, you need to get it in the air, and then carry it the whole way until you need it. It is not the price of the fuel that is expensive, it is having to carry it until you need it that is expensive. The fuel for take-off and climbing is used right away, so it is not expensive at all. The electricity bill for the winch would probably cost almost as much as the fuel you are saving.
The system seems perfectly feasible though. If it works on a small scale, for fighter jets and gliders, then why would it not work on a larger scale? I believe air traffic will slowly start to move towards electrically powered planes within the next few decades. For electric planes, a ground assisted take-off would be much more beneficial, or even essential. A winch could take the plane to above the clouds, and there it can fly to it's destination on solar power. If it had to get there itself, it would need huge battery packs.

while this would be ideal, using a 100% efficient solar panels providing the current ca. 5MW needed for an airplane during flight would need around 6000 squared meters of solar panel area.

If we take the Shockley–Queisser limit you will need 3 times more than that. But, then again the 5MW would include takeoff

@Ruud3.1415 There is still a lot of work to  be done, but i think we'll get there: https://www.theatlantic.com/photo/2016/07/flying-around-the-world-in-a-solar-powered-plane/493085/

"If it works on a small scale, for fighter jets and gliders, then why would it not work on a larger scale?": I can drop a 5 cm miniature car from 3 times it's length, it won't break. But I can't obviously do it with an actual car.

It works for fighter jets?  Which ones?  I can't begin to imagine the length of cable or winch engine needed to get a loaded 747 to cruise altitude...  And what if a 180 degree turn on course during climb is needed?  Tough luck I guess?  Sorry, I completely disagree that such an idea has any merit given the cruise altitudes of current long range airliners.


text:
Do fan blades use bell shaped lift distribution curves?
Seeing the organic curvature of some newer fans in turbofan engines, I'm curious if they use bell shaped lift distribution curves in their design, or something else all together.

I used incorrect terms, thanks for the clarification  The image shown by Peter Kämpf is what I was referring to.

The bell shaped lift distribution has been used on the Horten flying wings in order to reduce lift at the outer, rearward parts of the wing for stability and better controllability.
The fan blade of a jet engine has no need for a tailored reduction in lift towards the tips; instead it tries to maximize the thrust from the available area. The wavy shape is the result of a compromise which tries to sweep the outer, supersonic part of the blade aft and at the same time to minimize the torsion resulting from that sweep. Therefore, the blade is first swept forward and then aft.

RR Trent fan (picture source)

that picture ... that's a lot of mathematics and computer time  :O


text:
Is there a reason or code behind the Cessna model numbers?
Is there a story behind how Cessna model numbers were created?
E.g., models 170, 172, 180, 182, 206, 310, 336, etc.

Probably not. They clearly have a theme, but no one seems to know how it came about. There is a discussion at PoA about this topic. https://www.pilotsofamerica.com/community/threads/cessna-model-number-progression.12750/


text:
What physical tests are involved with verifying a wing spar is suitably constructed for a homebuilt aircraft, such as the piper cub kit from wicks?
During construction how is a spar tested to ensure it can adequately cope with bending and twisting forces of the completed structure, or is this performed after complete?

Are you asking as the builder, or the kit manufacturer?

As a builder. How does one know what they built is up to snuff

For a homebuilt a static load test is required. Whereas a full certification would require a loading to 1.5 the maximum load, this factor is reduced for experimental aircraft to 1.0. But performed it must be nonetheless.
For the test the aircraft is propped up upside down and sandbags (or water bottles) are placed on the wing until the calculated load is reached.

Structural test of the Electra One (picture source)

"But performed it must be..."  yoda would approve of this  :-)

I don't know if the economics are still sound, but when I helped a friend do the static load test on his Q2 we used bags filled with Portland cement.  The cement was cheaper than sand and being much more dense, the bags (of equivalent weight) were smaller and easier to arrange on the wing.

@Gerry: Cement bags are well suited (they will conform to the local surface just as well as sand), but sand bags can be individually filled to fine-tune the load. The fine dust produced by handling cement should be avoided near the airframe, or all holes should be taped shut.

@PeterKämpf  Agreed.  It was a long time ago, so I don't remember all the details.  The cement bags were very heavy (~60 lbs) and unwieldy.  We filled (outside) heavy duty garbage bags to 25 lbs each. Took care of the dust, made them easier to handle, and the standard weight simplified tracking and placement.

"Maximum load"...... You mean gross weight? @Peter Kämpf

@DavidTeahay: No, maximum lift. That is gross weight times maximum load factor.


text:
When would CAPT 3 or FO 3 ever be used on the A320?
On the Airbus A320, on the very top of the Pedestal and right below the bottom ECAM there are switches saying ATT HDG and the you can either switch it to CAPT 3 or FO 3 same for the other switches in that row AIR DATA, EIS DMC, and ECAM/ND XFR. I think they are for if the Captain or First Officer has custom gear or equipment. Please explain what each switch means, whats it's use, and when would they be used.

On what aircraft?

"ECAM" is a typical terminology of Airbus aircraft.
From this accident report found quickly while searching for "CAPT3":
The panel:

The explanations:

Normally all switch are positioned on NORM selection, mean that all of
  the source  are coming from co-location sources (i.e. system 1 for
  Captain, system 2 for F/O and  system 3 is standby).
In case of
  failure of either of the related system sources for Captain or F/O
  side, they  can alternate it by selecting the switch to either CAPT 3
  or F/O 3 (CAPT or F/O for  ECAM/ND XFR).

The switches are used to designate which input to connect to equipment or display.
When everything is fine, the NORM position links a left side input to a left side equipment, and a right side input to a right side equipment. If an input is not operational, then the switch can be used to select the third input for either the left side or right side equipment.
The equipment and sensors we are talking here are:

Attitude / Heading reference
Air data reference
Electronic Instrument System / Display Management Computers
ECAM and ND XFR used to manage where appear PFD, ND and SD.

Detailed example
The aircraft attitude and heading are sensed using cooperating inertial sensors and air sensors which are combined in an ADIRU. As the attitude determination is key to pilot the aircraft, in particular when autopilot is used, there are 3 redundant ADIRUs on a A320, one for the Capt (#1), one for the F/O (#2) and a backup one (#3). Each ADIRU #n receives data from its ADR #n unit (air data) and IR #n unit (inertial).
The NORM position allow to send the left ADIRU to the Capt panels and the right ADIRU to the F/O panels. This allows the Capt and the F/O to have separate data, and they can compare them in case of doubt. Same for the flight control computers behind Airbus fly-by-wire.
If ADR or IR fails on the Capt ADIRU (#1):

ATT HDG on CAPT 3 position replaces IR #1 by IR #3 on the ADIRU #1
AIR DATA on CAPT 3 position replaces ADR #1 by ADR #3 on the ADIRU #1

If the failure occurs on F/O side, corresponding F/O 3 positions would be used to replace IR #2 and ADR #2 by IR #3 and ADR #3.